1. Field of the Invention
The present invention relates to semiconductor devices mounted on a memory module used in a computer apparatus and the like and on other multi-chip module, and the like, a method of reducing unnecessary radiation noises in the semiconductor device, and a shield cover used in the semiconductor device.
2. Description of the Related Art
Recently, as the size of a notebook computer and the like is reduced in the field of a computer industry, a memory module as a semiconductor module used as a main memory is made more dense and smaller in size. The memory module is a memory device having semiconductor memory chips mounted thereon at high densities so that they operate at a high speed. The memory module is required to mount a larger number of memory chips in the limited mounting area thereof. Therefore, some products have chip size packages (hereinafter, abbreviated as CPS) having a smaller mounting area, tape carrier packages of a thinner type (hereinafter, abbreviated as TCP), and the like mounted thereon instead of conventional thin small out-line packages (TSOP).
On the other hand, as the performance of computers is enhanced, the bus speed of a main memory is increased. Further, as the operating frequency of a memory module is also increased, a serious problem arises in that electromagnetic noises are unnecessarily radiated from the memory module.
In the memory module on which the CSPs and the TCPs are mounted, semiconductor chips are sealed to a thin substrate or a film tape. However, since the semiconductor chips having such arrangement are less resistant to external stress and the like as well as require consideration to a heat dissipation effect. Consequently, the semiconductor chips must be covered with a shield cover formed of a suitable raw material such as metal.
Japanese Unexamined Patent Application Publication No. 2000-251463 (hereinafter, referred to as a patent document 1) discloses a typical example of the above technology. In the example, a shield sheet is formed by laminating at least an adhesive sheet and an aluminum sheet, bonded on the upper surfaces of memory chips and the upper surface of a printed circuit substrate so as to cover semiconductor memory chips to thereby electromagnetically shield the semiconductor memory chips as well as fix them to the printed circuit substrate. It is assumed that when this technology is used, since the electromagnetic noises radiated from the memory chips are shielded by the aluminum sheet and further the memory chips are fixed by the adhesive sheet, occurrence of faulty connection due to vibration can be suppressed and further a heat dissipation effect can be expected by causing the aluminum sheet to act as a heat sink.
However, the following problems still remain to effectively reinforce and electromagnetically shield the memory module having high density and high speed in correspondence to the recent computer technology.
First, in the technology disclosed in the above-mentioned patent document 1 in which the shield sheet is formed by laminating the adhesive sheet and the aluminum sheet and bonded on the upper surfaces of the memory chips and the upper surface of the printed circuit substrate so as to cover the semiconductor memory chips to thereby electromagnetically shield the memory chips as well as fix them to the printed circuit substrate, drawbacks arise in that distortion is liable to be caused by bonding due to the difference of the thermal expansion coefficients between the memory module and the shield sheet and that it is difficult to secure a heat dissipation property and a bonding surface area. In order to prevent these drawbacks, a shield method is required to more reduce an area in contact with a circuit substrate or memory chips.
Further, from the view point of mechanical strength, a memory module, in which memory chips are covered with a shield cover, namely, with a metal cover as disclosed in the patent document 1, has high mechanical strength, and thus this memory module is more preferable in mechanical strength than a memory module that is not covered with a shield cover.
However, as a result of studies, the inventors have found that, when a memory module operates at a high speed, electromagnetic noises are radiated more seriously when the memory module has a floating-state shield cover covering circuits than when the memory module has no shielding (that is, when circuits are not covered with the shield cover).
This is because the shield cover itself acts as a patch type antenna. It is assumed preferable to totally fix the shield to a reference potential in order to prevent this drawback. Specifically, in order to reduce the electromagnetic noises radiated from a shield cover, it is preferable to shield the noises radiated from a memory module. For this purpose, it is preferable to connect the entire portions of a circuit substrate in contact with the shield cover to the reference potential. However, it is difficult to actually apply this connection to a product. This is because it is necessary to form a ground pattern on the circuit substrate in correspondence to the contact surface of the shield cover to connect the shield cover to the reference potential in its entirety. However, the ground pattern cannot be form on the circuit substrate around the entire periphery of the shield cover because signal lines are formed on the substrate. Further, it is also difficult to cause the substrate to bring into contact with the entire periphery of the shield cover because of the structural problem of the shield cover. Moreover, even if the substrate is forcibly caused to bring into contact with the entire periphery of the shield cover, distortion is caused by the difference of thermal expansion coefficients due to the increased contact area thereof.
Further, as apparent from the above description, since the mechanism of noises radiated from the memory module is obscure, an effective installation method is not apparent. Accordingly, it has been found that when a memory module is operated in a frequency of, for example, 133 MHz or more in the state that the shield cover is grounded in an arbitrary portion, electromagnetic noises are radiated more than those in a floating state in a high frequency region of 666 MHz and the like as the harmonic component of the frequency.